Currently there exist conventional radio systems and trunking radio systems. Each system offers its users the ability to scan their talk groups.
In a typical conventional system, a communication unit scans between different frequencies in order to listen to the various group activity in the unit's scan list, an internally stored list of talkgroup/conventional frequencies. In addition, groups are assigned higher levels of receive priority by the scanning radio. During non-priority calls, the unit will check the higher priority channel frequencies for call activity at predetermined intervals, thereby causing small audio holes such as, for instance, 50 msec in length every 500 msec.
In a typical trunking system, a communication unit locks onto a control channel, and thus monitors the call assignments for a match to its list of groups to be scanned. If a match occurs, the unit will follow the instructions in that call assignment and go listen to the traffic on the trunked channels. In addition, groups to be scanned are assigned multiple levels of receive priority by the scanning radio. This, along with a priority monitor algorithm, allows the scanning unit to hear higher priority traffic. While listening to a lower priority call, the communication unit will be decoding special voice channel signalling from the trunked resource controller looking for an announcement that a higher priority group has subsequently started a call on another channel. If this new group is in the unit's scan list and is in a higher priority, it will leave the voice channel of the current call in favor of returning to the control channel. Once there, it will determine the actual assigned voice channel of the higher priority call and go over as a receiver. This priority monitor function is described in Jaime Andres Borras et al., "Trunked Communication System True Priority Channel Scan," U.S. Pat. No. 4,716,407, issued Dec. 29, 1987, which patent is hereby incorporated herein by reference.
With the growing demand for wide-area/state-wide systems there is a desire to have the ability to monitor various trunking talkgroups and conventional channel freqencies used by other agencies in a single radio personality. In such an arrangement, the scanning unit needs to access multiple systems in order to monitor all call activity of interest. In such a situation, it is important to minimize time delays. For example, with the scanning radio initially engaged in a call on a trunked channel, the radio must leave the trunked system, scan the conventional system, and return to the trunked system--all with minimum time delay.
As a result, there is a need for an improved scanning algorithm.